Externally heated protostellar cores in the Ophiuchus star-forming region

TL;DR

This study uses APEX 218 GHz observations to analyze molecular emission in protostars in Ophiuchus, revealing temperature variations due to external irradiation and identifying different gas tracers.

Contribution

It provides new measurements of molecular rotational temperatures in protostellar cores, highlighting the impact of external heating and differentiating gas tracers in the region.

Findings

H$_2$CO temperatures range from 16 K to 124 K, highest near IRAS 16293-2422.

c-C$_3$H$_2$ temperatures are consistently low (7-17 K).

External irradiation influences core temperatures and emission regions.

Abstract

We present APEX 218 GHz observations of molecular emission in a complete sample of embedded protostars in the Ophiuchus star-forming region. To study the physical properties of the cores, we calculate H$_2$CO and c-C$_3$H$_2$ rotational temperatures, both of which are good tracers of the kinetic temperature of the molecular gas. We find that the H$_2$CO temperatures range between 16 K and 124 K, with the highest H$_2$CO temperatures toward the hot corino source IRAS 16293-2422 (69-124 K) and the sources in the $\rho$ Oph A cloud (23-49 K) located close to the luminous Herbig Be star S 1, which externally irradiates the $\rho$ Oph A cores. On the other hand, the c-C$_3$H$_2$ rotational temperature is consistently low (7-17 K) in all sources. Our results indicate that the c-C$_3$H$_2$ emission is primarily tracing more shielded parts of the envelope whereas the H$_2$CO emission (at the angular scale of the APEX beam; 3600 au in Ophiuchus) mainly traces the outer irradiated envelopes, apart from in IRAS 16293-2422, where the hot corino emission dominates. In some sources, a secondary velocity component is also seen, possibly tracing the molecular outflow.